Thin film superconducting wire and superconducting cable conductor

ABSTRACT

A thin film superconducting wire with a copper plating thin film produced on a surface of a laminated structure is inferior in bending properties to a thin film superconducting wire having no copper plating thin film. Therefore, a thin film superconducting wire according to the present invention is a thin film superconducting wire including a laminated structure having a substrate, a buffer layer located on one of main surfaces of the substrate, and a superconducting layer located on a main surface of the buffer layer opposite to a main surface facing the substrate. The thin film superconducting wire further includes a copper plating thin film covering an outer periphery of the laminated structure, a residual stress within the copper plating thin film serving as a compression stress. The laminated structure may have a sputtered silver layer. A silver covering layer covering the outer periphery of the laminated structure may be further provided between the copper plating thin film and the laminated structure.

TECHNICAL FIELD

The present invention relates to a thin film superconducting wire and asuperconducting cable conductor, and more particularly to a thin filmsuperconducting wire that protects a superconducting layer through theuse of a residual stress within a thin film covering the superconductinglayer from outside, and to a superconducting cable conductor.

BACKGROUND ART

A thin film superconducting wire including a laminated structurecontaining a superconducting layer is conventionally known. The thinfilm superconducting wire is wound longitudinally in a spiral manneraround the surface of a cylindrical core material called a former, forexample, to thereby implement a superconducting cable conductor. Toachieve cost reduction, it is requested to reduce the outer diameter ofa cross section that longitudinally intersects the superconducting cableconductor, to reduce the outer diameter of a cross section thatlongitudinally intersects the former, and to wind the thin filmsuperconducting wire around the surface of the former. Accordingly, itis requested to wind the thin film superconducting wire while bending itwith a large curvature relative to its longitudinal direction. Thus,during the winding, a larger bending stress is applied to the radiallyouter side (toward the outer peripheral side) of the former in the thinfilm superconducting wire.

For example, a plurality of cable cores constituting a superconductingcable disclosed in Japanese Patent Laying-Open No. 2006-331893 (PatentDocument 1) indicated below has the following structure. Specifically,each cable core is configured to include a former positioned at thecenter of cable core, a first superconducting layer made of asuperconducting wire wound around the outer periphery of the former (asuperconducting wire with a superconducting filament covered by astabilizing metal such as silver), an insulation layer located outsidethe first superconducting layer, and a second superconducting layerlocated outside the insulation layer. At a cross section thatlongitudinally intersects the cable core, the second superconductinglayer located at the outer side has a diameter larger than that of thefirst superconducting layer located at the inner side. Thus, a bendingstress applied to the first superconducting layer while winding thecable core around the surface of the former or insulation layer, forexample, has a larger value than that applied to the secondsuperconducting layer. Therefore, executed in Patent Document 1 is aprocess of making the tensile strength of the second superconductinglayer to which a large bending stress is applied greater than thatapplied to the first superconducting layer by making the tensilestrength of a material constituting the second superconducting layergreater than that of a material constituting the first superconductinglayer (more specifically, by a method such as producing a covering layersuch as a copper plating layer as a reinforcing member that covers thesurface of the second superconducting layer, or bonding a tape layer asa reinforcing member to the second superconducting layer). The cablecore and the superconducting cable are thereby improved in bendingproperties.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Patent Laying-Open No. 2006-331893

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The inventors considered subjecting a thin film superconducting wireincluding a laminated structure containing a superconducting layer to ametal plating treatment (more specifically, a copper plating treatment)such that the surface of the laminated structure is covered for thepurpose of protecting the laminated structure and the like. A copperplating thin film produced on the surface of the laminated structure bythe copper plating treatment serves to protect the superconducting layeragainst erosion and the like. Since this copper plating thin film isconsidered as corresponding to the covering layer of the secondsuperconducting layer according to the aforementioned Patent Document 1,bending properties of the thin film superconducting wire with the copperplating thin film produced on its surface were studied assuming the thinfilm superconducting wire to be wound around a former. The resultsreveal that the thin film superconducting wire with the copper platingthin film produced on its surface was inferior in bending properties toa thin film superconducting wire having no copper plating thin film.

The present invention has been made to solve the above problems, and anobject of the present invention is to provide a thin filmsuperconducting wire with the copper plating thin film produced thereonand a superconducting cable having such thin film superconducting wire,which can be prevented from degrading in bending properties.

Means for Solving the Problems

Having studied intensely the aforementioned thin film superconductingwire with the copper plating thin film produced thereon, the inventorscompleted the present invention. More specifically, the inventorsfocused on a residual stress within the copper plating thin filmproduced on the thin film superconducting wire, which is not disclosedor suggested in Patent Document 1. Measuring the residual stress withinthe copper plating thin film reveals that the residual stress served asa tensile stress in a thin film superconducting wire having degradedbending properties.

In the case where the superconducting layer of the thin filmsuperconducting wire is located at a radially outer side of the formerwhen winding the thin film superconducting wire around the outerperiphery of the former, a tensile stress will be applied to thesuperconducting layer. Then, when a copper plating thin film to which atensile stress is applied as the aforementioned residual stress ispresent outside the laminated structure including the superconductinglayer as described above, an additional tensile stress resulting fromthe aforementioned residual stress within the copper plating thin filmwill be applied to the superconducting layer. This in result promotesdegradation in properties of the superconducting layer resulting fromthe added tensile stress, so that it is considered that the thin filmsuperconducting wire degrades in bending properties (e.g., a criticalbending diameter that allows bending while maintaining favorablesuperconducting properties increases).

Based on the above findings, a thin film superconducting wire accordingto the present invention includes a laminated structure having asubstrate, a buffer layer located on one of main surfaces of thesubstrate, and a superconducting layer located on a main surface of thebuffer layer opposite to the main surface facing the substrate. The thinfilm superconducting wire further includes a copper plating thin filmcovering an outer periphery of the laminated structure, a residualstress within the copper plating thin film serving as a compressionstress.

The above-described thin film superconducting wire according to thepresent invention has a structure in which the copper plating thin filmcovers substantially the entire surface of the outer periphery of thelaminated structure containing the superconducting layer. When windingthe thin film superconducting wire of such structure around the surfaceof the former, for example, in such a manner that the superconductinglayer faces the outside (opposite to the side facing the surface of theformer), then, a tensile stress is applied to the superconducting layerand the copper plating thin film located outside similarly to thesuperconducting layer, in such a direction that the length of the thinfilm superconducting wire increases. At this stage, if a residual stressin the direction of compression exists within the copper plating thinfilm, a compression stress as a residual stress acting in the oppositedirection to the above-described tensile stress associated with thewinding and the tensile stress associated with the winding cancel eachother out particularly within the copper plating thin film facing theoutside, so that the tensile stress associated with the winding can bereduced (or if the residual stress has a sufficiently large value, thetensile stress associated with the winding is canceled out, so that astress within the copper plating thin film serves as a compressionstress). Accordingly, the copper plating thin film facing the outsidecan be less likely to apply a large tensile stress to thesuperconducting layer (or by applying a compression stress to thesuperconducting layer, the tensile stress associated with the windingwithin the superconducting layer can be reduced in value). This inresult can suppress a mechanical deformation of the superconductinglayer constituting the thin film superconducting wire due to theaforementioned tensile stress and a reduction in critical current Icresulting from variations in composition arrangement, so that the thinfilm superconducting wire can be improved in electric properties andbending properties.

Preferably, in the above-described thin film superconducting wire, thelaminated structure further includes sputtered silver layers locatedrespectively on the other of the main surfaces of the substrate notfacing the buffer layer and on a main surface of the superconductinglayer not facing the buffer layer.

The sputtered silver layers serve as conducting layers through which acurrent conducts to the surface of the laminated structure during aplating treatment for producing the copper plating thin filmconstituting the thin film superconducting wire. Particularly, thesputtered silver layers are located respectively on the main surface ofthe superconducting layer (serving as the outermost surface) opposite toa main surface thereof facing the buffer layer and on the main surfaceof the substrate (serving as the outermost surface) opposite to a mainsurface thereof facing the buffer layer among the main surfacesconstituting the laminated structure (the largest main surfaces amongthe surfaces). Then, flowing a current to the surface of the laminatedstructure through the pair of sputtered silver layers allows the platingtreatment for producing the copper plating thin film to be conductedsmoothly.

In particular, the above-described sputtered silver layer located on themain surface of the superconducting layer (serving as the outermostsurface) acts as a protection layer for preventing the superconductinglayer, for example, from being eroded by a plating solution and thelike. Therefore, provision of the sputtered silver layer can reduce thelikelihood that the superconducting layer may be damaged by the platingtreatment.

Preferably, the above-described thin film superconducting wire furtherincludes a silver covering layer covering the outer periphery of thelaminated structure between the copper plating thin film and thelaminated structure. The silver covering layer is a thin film layerproduced before producing the copper plating thin film that covers theouter periphery of the laminated structure after producing the laminatedstructure, and serves to protect the superconducting layer and the likeagainst a copper plating solution used for the copper plating treatment.Therefore, provision of the silver covering layer can increaseflexibility in selection of the copper plating solution used for thecopper plating treatment.

In the thin film superconducting wire provided with the silver coveringlayer, some of an excessive current flowing through the superconductinglayer can be diverted and flown to the silver covering layer when adefect such as quenching occurs. This can prevent breakdown of thesuperconducting layer due to an excessive current flowing through thesuperconducting layer.

In the case where the superconducting cable, obtained by winding theabove-described thin film superconducting wire around the surface of theformer, for example, is wound such that the side at which thesuperconducting layer is located faces the outside, the tensile stressapplied to the copper plating thin film at the side where thesuperconducting layer is located can be smaller by the compressionstress as a residual stress within the copper plating thin film. Thiscan reduce the likelihood that a large tensile stress may be applied tothe superconducting layer. Since the thin film superconducting wire isthereby improved in electric properties and bending properties asdescribed above, the superconducting cable can be improved in electricproperties and bending properties.

Effects of the Invention

In the thin film superconducting wire according to the presentinvention, the residual stress within the copper plating thin filmcovering the outer periphery of the laminated structure serves as acompression stress. Accordingly, when winding the thin filmsuperconducting wire such that the side at which the superconductinglayer of the laminated structure is located faces the outside, thetensile stress applied to the copper plating thin film facing theoutside similarly to the superconducting layer can be smaller by thecompression stress which is the residual stress within the copperplating thin film. The thin film superconducting wire can thereby beimproved in electric properties and bending properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing a structure of a thin filmsuperconducting wire according to an embodiment of the presentinvention.

FIG. 2 is a perspective view showing a step of winding the thin filmsuperconducting wire according to the embodiment of the presentinvention in a pancake manner.

FIG. 3 is a plan view showing a state in which the thin filmsuperconducting wire according to the embodiment of the presentinvention is wound in a pancake manner.

FIG. 4 is an enlarged view of a region “IV” in FIG. 3.

FIG. 5 is a schematic sectional view showing a structure of a thin filmsuperconducting wire according to a first modification of the embodimentof the present invention.

FIG. 6 is a schematic sectional view showing a structure of a thin filmsuperconducting wire according to a second modification of theembodiment of the present invention.

FIG. 7 is a schematic sectional view showing a manner in which thesuperconducting thin film wire according to the embodiment of thepresent invention is wound around a former.

FIG. 8 is a schematic sectional view showing a structure of asuperconducting cable conductor according to the embodiment of thepresent invention.

FIG. 9 is a flow chart showing a method of manufacturing the thin filmsuperconducting wire according to the embodiment of the presentinvention.

FIG. 10 is a flow chart showing detailed steps in a step (S10) of theflow chart of FIG. 9.

FIG. 11 is a flow chart showing detailed steps in a step (S30) of theflow chart of FIG. 9.

FIG. 12 is a schematic sectional view showing a structure of the thinfilm superconducting wire according to the present embodiment.

MODES FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below withreference to the drawings. In the present embodiment, like referencecharacters denote like parts having like function, and the samedescription will not be repeated unless necessary.

FIGS. 1, 5, and 6 are schematic sectional views of a cross section takenin a direction intersecting the direction in which a thin filmsuperconducting wire according to the present embodiment extends.Accordingly, the direction intersecting the sheet of drawing is assumedto be the longitudinal direction of a thin film superconducting wire 10,and a superconducting current is assumed to flow through asuperconducting layer 5 in the direction intersecting the sheet ofdrawing. FIGS. 1, 5, and 6 show the cross section in rectangular shapewith a small difference between the length in the vertical direction andthat in the horizontal direction for easy understanding of the drawings,however, the vertical thickness of the cross section is actually muchsmaller than the horizontal width.

As shown in FIG. 1, thin film superconducting wire 10 according to theembodiment of the present invention has a long-length shape (tape-likeshape) having a rectangular cross section, and herein the main surfaceis implemented by a surface having a relatively larger area extending inthe longitudinal direction of the long-length shape. Thin filmsuperconducting wire 10 includes a laminated structure 20, a silvercovering layer 7 located so as to cover the outer periphery of laminatedstructure 20, and a copper plating thin film 9 located so as to coverthe outer periphery of silver covering layer 7. Laminated structure 20includes a substrate 1, a buffer layer 3 located on one of main surfacesof substrate 1, a superconducting layer 5 located on a main surface ofbuffer layer 3 (the upper main surface in FIG. 1) opposite to the mainsurface facing substrate 1, and sputtered silver layers 6. Sputteredsilver layers 6 are located on the other main surface of substrate 1(the lower main surface in FIG. 1) not facing buffer layer 3 and on amain surface of superconducting layer 5 (the upper main surface inFIG. 1) not facing buffer layer 3. More specifically, sputtered silverlayers 6 are located so as to sandwich substrate 1, buffer layer 3 andsuperconducting layer 5 from the upper and lower main surfaces. In otherwords, substrate 1, buffer layer 3, superconducting layer 5, andsputtered silver layers 6 described above constitute laminated structure20.

Substrate 1 preferably has a long-length shape (tape-like shape) havinga rectangular cross section, made of Hastelloy® or a nickel-based alloy,for example. A length that thin film superconducting wire 10 extends inthe extending direction is longer than or equal to 100 m, for example,and a length (width) thereof intersecting the extending direction isapproximately 10 mm, for example. To increase the current densityflowing through thin film superconducting wire 10, substrate 1preferably has a smaller cross-sectional area. However, if the thicknessof substrate 1 (in the vertical direction in FIG. 1) is too thin inorder to reduce the cross-sectional area of substrate 1, the strength ofsubstrate 1 may degrade. Accordingly, substrate 1 preferably has athickness of approximately 0.1 mm, for example.

If superconducting layer 5 is located on the main surface of substrate1, a polycrystalline thin film having an inferior orientation of crystalaxes in the direction along the main surface of substrate 1 will beproduced. In this case, it is difficult to increase the critical currentdensity (Jc) of the thin film superconducting wire obtained. Therefore,buffer layer 3 is located between substrate 1 and superconducting layer5. Buffer layer 3 is preferably implemented by a material such asGd₂Zr₂O₇ (oxide of Gd (gadolinium) and Zr (zirconium)), CeO₂ (ceriumoxide), or YSZ (yttria-stabilized zirconia). Implementing buffer layer 3by such a material facilitates aligning crystal axes of superconductinglayer 5 located on the main surface of buffer layer 3.

Superconducting layer 5 is a thin film layer through which asuperconducting current flows in thin film superconducting wire 10, andis preferably made of, for example, an yttrium-based (YBa₂Cu₃O_(x)) thinfilm that is a superconducting material. To improve the value ofcritical current Ic of the superconducting current flowing throughsuperconducting layer 5, superconducting layer 5 preferably has athickness greater than or equal to 0.1 μm and smaller than or equal to10 μm.

Sputtered silver layer 6 located on the main surface of superconductinglayer 5 is a silver thin film produced by a sputtering method, andpreferably has a thickness greater than or equal to 0.1 μm and smallerthan or equal to 50 μm. However, sputtered silver layer 6 can beimplemented by a silver thin film obtained by any method other thansputtering.

Sputtered silver layer 6 located on the main surface of superconductinglayer 5 is provided as a protection layer for preventing superconductinglayer 5 from being damaged by erosion and the like. As shown in FIG. 1,besides on the main surface of superconducting layer 5, sputtered silverlayer 6 is also provided on the lower main surface of substrate 1, sothat both sputtered silver layers 6 are located to sandwich substrate 1,buffer layer 3, and superconducting layer 5. Then, when conducting aplating treatment for producing copper plating thin film 9, an electrodeof a system of conducting the plating treatment and laminated structure20 can be easily brought into conduction through sputtered silver layers6.

Silver covering layer 7 is located so as to cover the outer periphery oflaminated structure 20 including sputtered silver layers 6, that is, soas to cover substantially the entire surface of the outermost surface oflaminated structure 20. Silver covering layer 7 is a silver thin filmproduced by plating, for example, and preferably has a thickness greaterthan or equal to 0.1 μm and smaller than or equal to 50 μm.

Locating silver covering layer 7 so as to cover substantially the entiresurface of the outermost surface of laminated structure 20 canfacilitate the plating treatment for producing copper plating thin film9 on laminated structure 20 including silver covering layer 7. In otherwords, copper plating thin film 9 is located on the outer surface ofsilver covering layer 7. When silver covering layer 7 has theabove-described thickness, the following effects can be achieved. Morespecifically, while immersing laminated structure 20 into a copperplating solution during the plating treatment, the surface ofsuperconducting layer 5 is not in direct contact with the copper platingsolution because the surface of superconducting layer 5 is covered bysilver covering layer 7. The presence of silver covering layer 7 canthereby reduce the likelihood that the surface and the inside ofsuperconducting layer 5 may be affected by, for example, erosion causedby the copper plating solution. Since the surface of laminated structure20 is covered by silver covering layer 7, there is no need to considerwhether superconducting layer 5 will be eroded or not when selecting thecopper plating solution, which can improve flexibility in selection ofthe copper plating solution used for the copper plating treatment.

In addition, providing thin film superconducting wire 10 with silvercovering layer 7 allows some of an excessive current flowing throughsuperconducting layer 5 to be diverted and flown to silver coveringlayer 7 when a defect such as quenching occurs in superconducting layer5, for example. This can prevent breakdown of superconducting layer 5due to an excessive current flowing through superconducting layer 5.

Copper plating thin film 9 is located so as to cover the outer peripheryof above-described silver covering layer 7, that is, so as to coversubstantially the entire surface of the outermost surface of silvercovering layer 7. Copper plating thin film 9 is a copper thin filmproduced by plating, and preferably has a thickness greater than orequal to 0.1 μm and smaller than or equal to 50 μm.

In copper plating thin film 9 of thin film superconducting wire 10, aresidual stress existing therein serves as a compression stress. Morespecifically, it is in a state where force is applied to a region in thedirection along which copper plating thin film 9 extends, for example,that is, in the longitudinal direction of thin film superconducting wire10, the force being applied in such a direction that the longitudinallength of thin film superconducting wire 10 is reduced.

By way of example, as shown in FIGS. 2 and 3, consider the case in whichthin film superconducting wire 10 is wound in a pancake manner (like acoil). More specifically, thin film superconducting wire 10 is woundaround reel 15 located on a flat plate 14 in the direction of a rotatingarrow (direction A) on reel 15 shown in FIG. 2, to constitute a coil100. At this state, winding is performed such that superconducting layer5 is located at the outer side of winding of one thin filmsuperconducting wire 10 surrounded by a dotted line in FIG. 3 (at theupper side in FIG. 1) as shown in an enlarged view of FIG. 4, and suchthat substrate 1 is located at the inner side of winding of that thinfilm superconducting wire 10 (at the lower side in FIG. 1). Thus, inthin film superconducting wire 10 wound as shown in FIGS. 2 and 3, theside at which substrate 1 is located faces reel 15, and the side atwhich superconducting layer 5 is located faces the outside of winding,not facing reel 15.

In this case, as shown in FIG. 4, a tensile stress is applied to theouter side of winding of thin film superconducting wire 10 such that thelength extending in the longitudinal direction is longer than at theinner side of winding. Accordingly, a tensile stress 11 shown in FIG. 4is applied to the main surface of copper plating thin film 9 located atthe outer side of winding at the side where superconducting layer 5 islocated. Then, a stress caused by tensile stress 11 is also applied tosuperconducting layer 5 located in proximity to copper plating thin film9 to which tensile stress 11 is applied. However, as described above, acompression stress 12 shown in FIG. 4 exists as a residual stress incopper plating thin film 9 of thin film superconducting wire 10. Sincetensile stress 11 and compression stress 12 act in opposite directionsto cancel each other out, the magnitude of tensile stress 11 is reducedby compression stress 12 (or when compression stress 12 is sufficientlygreat, tensile stress 11 is completely canceled out).

For example, when winding thin film superconducting wire 10, an increasein the tensile stress acting on copper plating thin film 9 located onthe main surface at the outer side of winding may cause stressconcentrations locally in superconducting layer 5 located at the outerside of winding and the like due to the increased tensile stress incopper plating thin film 9. If a great stress is applied tosuperconducting layer 5, critical current Ic in superconducting layer 5decreases in value. In addition, if superconducting layer 5 is broken asa result of the occurrence of stress concentrations and the like in thesuperconducting layer, a superconducting current can no longer be flownthrough superconducting layer 5.

In the case of thin film superconducting wire 10 according to thepresent invention, however, tensile stress 11 within copper plating thinfilm 9 caused by winding decreases because of the presence ofcompression stress 12 as a residual stress within copper plating thinfilm 9 acting in the opposite direction to tensile stress 11. This cansuppress degradation of superconducting layer 5 resulting from tensilestress 11, and suppress reduction in the superconducting current(critical current Ic) that can be flown through superconducting layer 5.Therefore, thin film superconducting wire 10 can be improved in electricproperties and bending properties. More specifically, thin filmsuperconducting wire 10 has a large critical current Ic and offers highperformance (with a great curvature which is a bending limit).

A thin film superconducting wire 30 shown in FIG. 5 is a modification ofabove-described thin film superconducting wire 10 shown in FIG. 1, andbasically has an aspect similar to that of thin film superconductingwire 10. However, thin film superconducting wire 30 is not provided withsilver covering layer 7.

The aspect of thin film superconducting wire 30 is different from thatof thin film superconducting wire 10 merely in the following point. Thatis, with respect to other elements, thin film superconducting wire 30has an aspect similar to that of thin film superconducting wire 10. Morespecifically, a residual stress within copper plating thin film 9 ofthin film superconducting wire 30 also serves as a compression stresssimilarly to copper plating thin film 9 of thin film superconductingwire 10. This achieves the effect of reducing a tensile stress by theaction of the compression stress even when the tensile stress is appliedto copper plating thin film 9 at the side where superconducting layer 5is located, thereby preventing superconducting layer 5 from degrading.Therefore, the thin film superconducting wire according to the presentembodiment may be implemented by the aspect of thin film superconductingwire 30.

A silver crystal is a stable cubic, whose melting point is as high as961° C. However, silver has a characteristic that, when heated, easilystarts being softened even at low temperatures below its melting point.For example, in the case of providing thin film superconducting wire 10,silver covering layer 7 is heated by conduction through silver coveringlayer 7 in the process of conducting the plating treatment for producingcopper plating thin film 9 after producing silver covering layer 7.Then, silver covering layer 7 may be softened by heating and may bedeformed.

In thin film superconducting wire 30, the absence of silver coveringlayer 7 can avoid defects associated with softening of silver coveringlayer 7 during the above-described plating treatment. In the step ofproducing thin film superconducting wire 30 shown in FIG. 5, sputteredsilver layers 6 can be utilized to facilitate conduction betweenlaminated structure 20 and the electrode of the system of conducting theplating treatment for producing copper plating thin film 9.

A thin film superconducting wire 50 shown in FIG. 6 is a modification ofabove-described thin film superconducting wire 30 shown in FIG. 5, andbasically has an aspect similar to that of thin film superconductingwire 30. However, thin film superconducting wire 50 is provided withsputtered silver layers 6 also in the direction intersecting the mainsurface of laminated structure 20 (in the vertical direction in FIG. 6,i.e., on the side surfaces of laminated structure 20) in addition tothose along the main surface of laminated structure 20.

The aspect of thin film superconducting wire 50 is different from thatof thin film superconducting wire 30 merely in the following point. Thatis, with respect to other elements, thin film superconducting wire 50has an aspect similar to that of thin film superconducting wire 30. Morespecifically, a residual stress within copper plating thin film 9 ofthin film superconducting wire 50 also serves as a compression stresssimilarly to copper plating thin film 9 of thin film superconductingwire 10. This achieves the effect of reducing a tensile stress by theaction of the compression stress even when the tensile stress is appliedto copper plating thin film 9 at the side where superconducting layer 5is located, thereby preventing superconducting layer 5 from degrading.Therefore, the thin film superconducting wire according to the presentembodiment may be implemented by the aspect of thin film superconductingwire 50.

By providing sputtered silver layers 6 also in the directionintersecting the main surface of laminated structure 20 (in the verticaldirection in FIG. 6) in addition to those along the main surface oflaminated structure 20, sputtered silver layers 6 are configured tocover substantially the entire surface of the outer periphery oflaminated structure 20 similarly to silver covering layer 7. Therefore,similarly to silver covering layer 7, they serve as protection layersfor reducing the likelihood that a defect such as erosion ofsuperconducting layer 5 by a copper plating solution may occur duringthe plating treatment for producing copper plating thin film 9.

Further, similarly to above-described silver covering layer 7, sputteredsilver layers 6 in thin film superconducting wire 50 can play the role(as a stabilization layer) to divert and flow some of an excessivecurrent flowing through superconducting layer 5 when a defect such asquenching occurs in superconducting layer 5, for example. This canprevent breakdown of superconducting layer 5 due to an excessive currentflowing through superconducting layer 5.

In the case of producing sputtered silver layers 6 on the entireperiphery of laminated structure 20 similarly to silver covering layer 7(cf. FIG. 1) as in thin film superconducting wire 50, it is preferableto form sputtered silver layers 6 thicker than sputtered silver layers 6in thin film superconducting wire 10, for example. More specifically,the thickness is preferably greater than or equal to 0.5 μm and smallerthan or equal to 50 μm.

A superconducting cable conductor obtained by using above-described thinfilm superconducting wire 10, 30, or 50 will now be described.

In the case of using thin film superconducting wire 10, for example,thin film superconducting wire 10 is wound in a spiral manner around theouter peripheral surface of a cylindrical former 60, as shown in FIG. 7.In this manner, a plurality of superconducting wires 10 are laminated inmultiple layers on former 60 to constitute a superconducting cableconductor 70 shown in FIG. 8. As described above, thin filmsuperconducting wire 10 is wound such that superconducting layer 5 islocated at the outer side of winding of each thin film superconductingwire 10 (at the upper side in FIG. 1), and such that substrate 1 islocated at the inner side of winding of thin film superconducting wire10 (at the lower side in FIG. 1). Thus, in thin film superconductingwire 10 wound as shown in FIGS. 7 and 8, the side at which substrate 1is located faces the outer peripheral surface of former 60, and the sideat which superconducting layer 5 is located faces the outside ofwinding, not facing former 60. Then, as described above, tensile stress11 (cf. FIG. 4) applied to the main surface of copper plating thin film9 at the side where superconducting layer 5 of thin film superconductingwire 10 is located is reduced by compression stress 12 (cf. FIG. 4)within copper plating thin film 9. This reduces the stress applied tosuperconducting layer 5 of thin film superconducting wire 10, which canavoid defects such as a reduction in critical current Ic due todegradation and breakdown of thin film superconducting wire 10, therebyimproving the electric properties and bending properties.

As shown in FIG. 8, thin film superconducting wires 10 are wound suchthat the respective layers are in alternately different directions: forexample, a first layer 10 a is wound clockwise in the drawing; a secondlayer 10 b wound counterclockwise in the drawing; a third layer 10 cwound clockwise in the drawing; and a fourth layer 10 d woundcounterclockwise in the drawing. The directions of winding first layer10 a to fourth layer 10 d are not limited to these, but may be wound inany direction. For example, first layer 10 a and second layer 10 b maybe wound clockwise in the drawing, and third layer 10 c and fourth layer10 d may be wound counterclockwise in the drawing, or first layer 10 ato fourth layer 10 d may all be wound in a single direction.

A method of producing the thin film superconducting wire according tothe present embodiment described above will now be described. As shownin the flow chart of FIG. 9, a step (S10) of producing a laminatedstructure is executed first. More specifically, it is a step ofproducing laminated structure 20 of each of the thin filmsuperconducting wires shown in FIGS. 1, 5 and 6. Further specifically,the step (S10) of producing the laminated structure includes a step(S11) of preparing a superconducting layer and a step (S12) of producinga sputtered silver layer, as shown in the flow chart of FIG. 10.

The step (S11) of preparing a superconducting layer will now bedescribed specifically. To prepare superconducting layer 5, a structurein which substrate 1 and buffer layer 3 are laminated is prepared first.As a method of producing buffer layer 3 on one of the main surfaces ofsubstrate 1, an IBAD (Ion Beam Assisted Deposition) method, a PLD(Pulsed Laser Deposition) method, or sputtering can be employed. Usingthese methods, superconducting layer 5 located on the main surface ofbuffer layer 3 can be improved in crystal orientation. Buffer layer 3may be a single layer, or may be implemented by laminating a pluralityof layers.

Superconducting layer 5 is then produced on the main surface of bufferlayer 3. Superconducting layer 5 is preferably implemented by a thinfilm made of an yttrium-based (YBa₂Cu₃O_(x)) superconductor, forexample, using the PLD method, a high-frequency sputtering method, or aMOD (Metal Organic Deposition) method.

In the next step (S12) of producing the sputtered silver layers,sputtered silver layers 6 are preferably produced by sputtering, but canbe implemented by silver thin films produced by any method other thansputtering, as described above. In the case of providing thin filmsuperconducting wire 50 shown in FIG. 6, the step of producing sputteredsilver layers 6 on the main surfaces of laminated structure 20 isfollowed by a step of producing sputtered silver layers 6 on the sidesurfaces of laminated structure 20. In the step of producing sputteredsilver layers 6 on the side surfaces of laminated structure 20, anymethod can be employed such as locating the main surfaces of laminatedstructure 20 in an inclined manner relative to a target material ofsputtering such that the end faces of laminated structure 20 face thetarget material of sputtering. Sputtered silver layers 6 may be producedsimultaneously both on the main surfaces and side surfaces of laminatedstructure 20. In this case, the inclination angle of the main surfacesof laminated structure 20 relative to the target material may beadjusted appropriately such that sputtered silver layers 6 are producedsimultaneously both on one of the main surfaces (e.g., the outermostsurface of superconducting layer 5) and one of the side surfaces oflaminated structure 20.

In the case of providing thin film superconducting wire 10, a step (S20)of producing a silver covering layer is then executed. Morespecifically, it is a step of producing silver covering layer 7 shown inFIG. 1, which is preferably implemented by a vacuum deposition method ora sputtering method. The step (S20) of producing a silver covering layeris omitted in the case of providing thin film superconducting wire 30 or50.

Then, copper plating thin film 9 is produced in a step (S30) ofproducing a copper plating thin film. More specifically, the step (S30)of producing a copper plating thin film includes a step (S31) ofpreparing a copper plating solution, a step (S32) of adding an additiveand a step (S33) of conducting a plating treatment, as shown in a flowchart of FIG. 11.

The copper plating solution used for immersing laminated structure 20therein to obtain copper plating thin film 9 is preferably implementedby a solution obtained by dissolving copper sulfate in a sulfuric acidsolution or a solution obtained by dissolving copper pyrophosphate inliquid ammonia, for example. In order for the residual stress withincopper plating thin film 9 to serve as a stress in the compressiondirection, the electrolyte obtained by dissolving copper pyrophosphatein liquid ammonia is more preferable. To prevent erosion of the surfaceof superconducting layer 5 constituting laminated structure 20 duringimmersion of laminated structure 20, the use of the solution obtained bydissolving copper sulfate in sulfuric acid is more preferable.

The copper plating solution prepared as described above is subjected tothe step (S32) of adding an additive as shown in FIG. 11. Morespecifically, it is a step of adding an additive to the copper platingsolution in order to improve the smoothness and glossiness of thesurface of copper plating thin film 9 obtained.

The additive can be implemented by an organic compound-based materialsuch as polyethylene glycol or a surfactant. However, the smoothness andglossiness of the surface of copper plating thin film 9 can be greatlyimproved by using thiourea as the additive. The use of thiourea as theadditive also allows the residual stress within copper plating thin film9 to serve as a stress in the compression direction. Accordingly, it ispreferable to add thiourea to a solution obtained by dissolving coppersulfate in sulfuric acid, for example (e.g., a solution having a coppersulfate concentration greater than or equal to 60 g/l (litter) andsmaller than or equal to 150 g/l and a sulfuric acid concentrationgreater than or equal to 100 g/l and smaller than or equal to 220 g/l)such that thiourea has a concentration greater than or equal to 8 ppmand smaller than or equal to 12 ppm. This allows the residual stresswithin copper plating thin film 9 to serve as a stress in thecompression direction even when the copper plating solution isimplemented by the solution obtained by dissolving copper sulfate insulfuric acid.

Then, in the step (S33) of conducting a plating treatment, laminatedstructure 20 is immersed into the copper plating solution, with theadditive having been added thereto, constituting the system ofconducting the plating treatment. Sputtered silver layers 6 or silvercovering layer 7 of laminated structure 20 is then connected to anelectrode (cathode) for the plating treatment placed in the copperplating solution. Utilizing an electrolytic phenomenon caused byapplying a voltage to electrodes (cathode and anode) to flow a current,copper plating thin film 9 is produced on the surface of laminatedstructure 20 connected to the cathode. The current flowing through thecathode at this stage can have a value greater than or equal to 1 A/dm²and smaller than or equal to 10 A/dm², and more preferably 5 A/dm², forexample.

In the step (S33) of conducting the plating treatment, when implementingthe copper plating solution by a solution obtained by dissolving coppersulfate in sulfuric acid, for example, the plating bath preferably has atemperature higher than or equal to 20° C. and lower than or equal to30° C., and when implementing the copper plating solution by a solutionobtained by dissolving copper pyrophosphate in liquid ammonia, forexample, the plating bath preferably has a temperature higher than orequal to 50° C. and lower than or equal to 60° C.

By executing the above-described respective steps, thin filmsuperconducting wire 10, 30, or 50 having copper plating thin film 9whose internal residual stress serves as a compression stress can beobtained.

FIRST EXAMPLE

A thin film superconducting wire 90 shown in FIG. 12 with copper platingthin film 9 produced using a plating solution as a copper platingsolution obtained by dissolving copper sulfate in a sulfuric acid mediumwas produced, and subjected to a test to examine a residual stresswithin copper plating thin film 9 of thin film superconducting wire 90.

Herein, three kinds of copper plating solutions A, B, and C wereprepared in accordance with concentrations of copper sulfatepentahydrate (CuSO₄.5H₂O) dissolved in the sulfuric acid medium. Silvercovering layer 7 covering substantially the entire surface of the outerperiphery of laminated structure 20 of thin film superconducting wire 10shown in FIG. 1 was located to be connected to the cathode of the systemof conducting the plating treatment, so that the plating treatment byelectrolysis was conducted. Besides adding thiourea as the additive, asmall amount of chloride ion was added to copper plating solutions A, B,and C so as to prevent the copper plating solutions from erodinglaminated structure 20.

A target material of the plating treatment for producing copper platingthin film 9 was prepared as indicated below. As shown in FIG. 12, CeO₂in 0.1 μm thickness, YSZ in 0.3 μm thickness and CeO₂ in 0.1 μmthickness as a first buffer layer 3 a, a second buffer layer 3 b, and athird buffer layer 3 c, respectively, were sequentially produced by a RFsputtering method on one of the main surfaces of a tape-like substrateas substrate 1 made of nickel-tungsten alloy having a width of 10 mm anda thickness of 80 μm. These first buffer layer 3 a, second buffer layer3 b, and third buffer layer 3 c constitute buffer layer 3. The surfaceof a structure in which 2-μm-thick superconducting layer 5 made of RE123(REBa₂Cu₃O_(7-δ)) had been produced on third buffer layer 3 c using thePLD method was subjected to a DC sputtering method to produce silvercovering layer 7. Silver covering layer 7 had a thickness of 8 μm on themain surface of superconducting layer 5, a thickness of 2 μm on the mainsurface of substrate 1 and a thickness of 3 μm on both side surfacesalong the direction of lamination.

Copper plating thin film 9 was produced under various conditions shownin Table 1 in which these concentrations of copper sulfate, currentdensity flown through the cathode in the plating treatment, and time forthe plating treatment were varied. As a result, 20-μm-thick copperplating thin film 9 was obtained. The direction and magnitude of aresidual stress within copper plating thin film 9 of thin filmsuperconducting wire 10 thus obtained were measured, the results ofwhich are given in Table 1 below. It is to be noted that the residualstress was evaluated using an X-ray stress measurement device.

TABLE 1 Amount of Copper Concentration Amount of Magnitude CopperSulfate in of Sulfuric Chloride Current Plating Plating Bath Directionof of Residual Plating Sulfuric Acid Acid Ions Density Time TemperatureResidual Stress Solution (g/L) (g/L) (ppm) Additive (A/dm²) (min) (° C.)Stress (MPa) A 75 190 75 Thiourea 5 18 25 Tension 3 10 ppm 10 9 Tension5 B 100 150 10 9 Tension 6 C 150 100 10 9 Compression 4 15 6 Tension 35

Table 1 reveals the result that copper plating thin film 9 of thin filmsuperconducting wire 10 obtained by conducting the plating treatmentusing copper plating solution C with a current density of 10 A/dm² for aplating time of 9 min had an internal residual stress of 4 MPa in thedirection of compression.

SECOND EXAMPLE

Thin film superconducting wire 90 with copper plating thin film 9produced using an electrolyte as a copper plating solution obtained bydissolving copper pyrophosphate in liquid ammonia was produced, andsubjected to a test to examine a residual stress within copper platingthin film 9 of thin film superconducting wire 90.

Herein, a copper plating solution D obtained by dissolving copperpyrophosphate in liquid ammonia was prepared. A target material of theplating treatment was implemented by a material having the samestructure as that used in above-described First Example. Silver coveringlayer 7 covering substantially the entire surface of the outer peripheryof laminated structure 20 of thin film superconducting wire 90 shown inFIG. 12 was located to be connected to the cathode of the system ofconducting the plating treatment, so that the plating treatment byelectrolysis was conducted. Besides copper pyrophosphate, potassiumpyrophosphate which was potassium salt having a high solubilitycontaining phosphate ions was dissolved in copper plating solution D.This is for ensuring the amount of phosphate ions dissolved in theelectrolyte of copper plating solution D.

Copper plating thin film 9 was produced using copper plating solution Dthus prepared under conditions shown in Table 2 below, similarly toFirst Example. The direction and magnitude of a residual stress withincopper plating thin film 9 of thin film superconducting wire 90 thusobtained were measured, the results of which are given in Table 2 below.

TABLE 2 Amount of Amount of Copper Potassium Concentration MagnitudeCopper Pyrophosphate Pyrophosphate of Liquid PH of Current PlatingPlating Bath Direction of of Residual Plating in Liquid in LiquidAmmonia Electro- P Ratio of Density Time Temperature Residual StressSolution Ammonia (g/L) Ammonia (g/L) (mL/L) lyte Electrolyte (A/dm²)(min) (° C.) Stress (MPa) D 84 350 2 8.5 7.5 5 18 55 Compression 300

The P ratio in Table 2 is a value representing a weight ratio ofpyrophosphate ions to copper ions in the electrolyte of copper platingsolution D. Table 2 reveals the result that copper plating thin film 9of thin film superconducting wire 90 obtained by conducting the platingtreatment using copper plating solution D with a current density of 5A/dm² for a plating time of 18 min had an internal residual stress of300 MPa in the direction of compression.

Therefore, it can be said that, when implementing the copper platingsolution by a copper pyrophosphate solution, copper plating thin film 9having a residual stress in the direction of compression can be obtainedwithout adding an additive.

It should be construed that the embodiments disclosed herein are by wayof illustration in all respects and not intended to be limiting. It isintended that the scope of the present invention is defined by claims,not by the description above, and includes all modifications andvariations equivalent in meaning and scope to the claims.

Industrial Applicability

The present invention is particularly favorable as a technique ofimproving a thin film superconducting wire in electric properties andbending properties.

Description of the Reference Signs

1 substrate; 3 buffer layer; 3 a first buffer layer; 3 b second bufferlayer; 3 c third buffer layer; 5 superconducting layer; 6 sputteredsilver layer; 7 silver covering layer; 9 copper plating thin film; 10,30, 50, 90 thin film superconducting wire; 10 a first layer; 10 b secondlayer; 10 c third layer; 10 d fourth layer; 11 tensile stress; 12compression stress; 14 flat plate; 15 reel; 20 laminated structure; 60former; 70 superconducting cable conductor; 100 coil.

The invention claimed is:
 1. A thin film superconducting wirecomprising: a laminated structure including, a substrate, a buffer layerlocated on one of main surfaces of said substrate, and a superconductinglayer located on a main surface of said buffer layer opposite to a mainsurface facing said substrate; and a copper plating thin film coveringan outer periphery of said laminated structure, a residual stress withinsaid copper plating thin film being a compression stress in a directionalong the longitudinal extension of the thin film superconducting wirewhen the wire is in an unbent state, wherein the copper plating film isplated to the outer periphery by a solution obtained by dissolvingcopper sulfate pentahydrate in a sulfuric acid, wherein the solution hasa copper sulfate concentration greater than or equal to 60 g/l andsmaller than or equal to 150 g/l and a sulfuric acid concentrationgreater than or equal to 100 g/l and smaller than or equal to 220 g/l,and a thiourea concentration greater than or equal to 8 ppm and smallerthan or equal to 12 ppm.
 2. The thin film superconducting wire accordingto claim 1, wherein said laminated structure further includes sputteredsilver layers located respectively on the other of the main surfaces ofthe substrate not facing said buffer layer and on a main surface of saidsuperconducting layer not facing said buffer layer.
 3. The thin filmsuperconducting wire according to claim 1, further comprising a silvercovering layer covering the outer periphery of said laminated structurebetween said copper plating thin film and said laminated structure. 4.The thin film superconducting wire according to claim 1, wherein thecompression stress is greater than or equal to 4 MPa and is less than orequal to 300 MPa.
 5. A superconducting cable conductor comprising thethin film superconducting wire as defined in claim 1.